Fatigue Life Degradation Model for Anchor-Induced Indentation Damage in the Touchdown Zone of Unbonded Flexible Risers

Waves, currents, and platform motions subject the touchdown zone (TDZ) of unbonded flexible risers to sustained cyclic loading, making it highly susceptible to fatigue failure. Local indentation damage caused by dropped anchors alters the stress state of the pressure armor layer and accelerates fatigue accumulation, yet a quantitative relationship between indentation depth and residual fatigue life is still lacking. To address this issue, a global–local coupled fatigue assessment approach is proposed. A three-dimensional Hall anchor–riser–seabed finite element model was established in Abaqus/Explicit to evaluate local responses under different indentation depths. Combined with OrcaFlex global analysis, rainflow counting, and Miner’s rule, residual fatigue life was evaluated and a fatigue life degradation model based on the indentation-induced stress amplification index (Kd) was developed. Plastic yielding begins at an indentation depth of approximately 1.82 mm, while residual fatigue life decreases to about 10% of the reference case at 9 mm (representing a relative degradation trend rather than an absolute fatigue-life prediction. The Kd–Lf power-law model achieves R2 = 0.9999, with an exponent of 3.026, close to the DNV-RP-C203 Class D S-N curve slope (m = 3.0). The proposed model provides a practical tool for integrity assessment and residual fatigue life prediction of anchor-damaged unbonded flexible risers.